Lattice Defects Generated by Cyclic Thermomechanical Loading of Superelastic NiTi Wire

Cyclic instability of stress–strain–temperature functional responses of NiTi is presumably due to the plastic deformation accompanying martensitic transformation proceeding under external stress. In order to obtain systematic experimental evidence on this, we have performed series of cyclic thermomechanical loading tests (10 cycles) on superelastic NiTi wires with nanocrystalline microstructure, evaluated accumulated unrecovered strains and analysed permanent lattice defects created during the cycling by TEM. The accumulated unrecovered strains and density of lattice defects increased with increasing temperature and stress, at which the forward and/or reverse transformation proceeded. It did not correlate with the temperature and stress applied in the test as such. If the martensitic transformation proceeded at low stress (<100 MPa), the cyclic stress–strain–temperature responses of the wire were found to be almost stable (only marginal accumulated unrecovered strain and few isolated dislocation loops and segments were generated during the thermomechanical cycling). This was the case in thermal cycling at low stresses or in cyclic shape memory test. If the forward and/or reverse martensitic transformation proceeded under large external stress (>250 MPa), the responses were very unstable (large accumulated unrecovered strains and high density of dislocations and deformation bands). A scheme allowing for estimating the cyclic instability of functional behaviours of various NiTi wires in wide range of thermomechanical loading tests was introduced.